First off, you’ll want to find the width of the entire layout, so you need to identify that container. On the layout I am currently using, that container has an ID of wrapper – which is a pretty common name and might very well be what your theme uses as well.
Using Firebug, I am able to determine that the width of my wrapper element is 960 pixels, so I want to make my content area fill that space.
The next thing I’m going to do is identify the width of my content column. In my current layout that is contained within an element called content (also a common practice). The current width is 550pixels, so I need to increase that to 960.
Something to keep in mind here is that my current layout doesn’t require any margins, padding, or borders on it’s content area. Your theme could be different, and if so you need to factor in those values.
Remember, if you select your content element in the left hand column of Firebug, you’ll be able to see the padding (purple) and margins (yellow) highlighted on the page.
If your content area has padding/margins, you need to know what they are, and subtract the total value (the sum of both sides!) from your final width.
Here’s an example for you. We already know my wrapper area is 960 pixels across, and my content is 550 pixels. If my content had a 10 pixel margin on each side, I’d want to change my width to 940 pixels (960-10(left)-10(right) ).
If parts of that still don’t make sense, I’d definitely recommend reading a little about the CSS Box Model – that will help clarify what you need to allow space for and how margins and padding work.